electromagnetics of the obscure or “been there, done that macnae deep em.pdf · electromagnetics...

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Electromagnetics of the Obscureor “Been there, done that “

James Macnae

March 2011

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RMIT University©2011 Applied Sciences / Geophysics 2

History: As you get older as an explorationist you have generally done a lot of things and (hopefully) learnt something along the way.

Minerals where “direct detection” is possible that I have been paid to consult on, explore for or found by accident with electromagnetics

Geothermal fluidsSaline fluids (Tailings leakage, agriculture, coastal intrusion,

paleochannels)

Cindered CoalClays (paleochannels,

alteration)

PlasticGraphite (DC powerlinegrounds and economic

support for drillers)Lead-Zinc sulphides

ChromiteIron Ore (Grade

estimation through susceptibility)

Nickel Sulphides

Native CopperManganese OxidesCopper Sulphides

ObscureNot uncommonCommon

Note: These examples are taken from personal experience

The talk will focus on the obscure examples in red, with a brief mention of those in blue

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History: As you get older as an explorationist you have generally done a lot of things and (hopefully) learnt something along the way.

• Minerals where “indirect detection” is possible that I have been paid to consult on, explore for or found by accident with electromagnetics

Oil/Gas

Depth to bedrockUranium (Graphite or

paleochannel or alteration association)

ObscureNot uncommonCommon

GravelFresh Water

Opals (Silica)Platinum / PalladiumGold

Chromite

Cassiterite (Tin)

Pyrrhotite association at Renison Bell

Diamond (Kimberlitepipe mapping)

Ignoring culture: powerlines, pipelines, fences, sheds, railways, communications etc

My third ever survey as a raw geophysicist with Geoterrex was for Massive Sulphide Exploration in the North West Cape of South Africa, following up an airborne Input survey. In this survey we measured very unusual responses that Bob Keith recognised as being Kimberlites. Geoterrex staff knew about airborne kimberlite responses, but were unable to talk about them due to confidentiality constraints. The obscure responses were indeed kimberlites, and led to the Macnae (1979) Geophysics paper on “Kimberlites and exploration geophysics”which included field data and some new scale modelling designed to help interpret edge effects.

When I started in 1972, diamonds were in the obscure column, as was Uranium

Opal detection was the subject of a patent dispute I was involved with as an expert witness, where the novelty of the “invention” was to the claim to use 1 to 3 m station spacings rather than 25 to 50 m station spacings typical in mineral exploration.

Precious metals platinum/palladium can be associated with very poor conductors at chilled margin of ultrabasic intrusions

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Outline

• History (history)

• Electromagnetics oversimplified

• The EM response of obscure targets

– Chromite

– Cassiterite (tin)

• Obscure EM Response Interpretation and Misinterpretation

– Manganese and altimeters

– Sulphides and Native Copper

• Practical Example

There is no need to comment on an outline, except that I could have included interesting stuff on diamonds, uranium, oil/gas

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Electromagnetics Oversimplified

• EM response controlled by

– Conductivity and structure of target and its surroundings

– Dielectric Permittivity (Obscure for standard EM, mainstream for GPR)

– Magnetic susceptibility (Obscure for standard EM, mainstream formagnetics)

– Frequency dependence of conductivity (Obscure for standard EM, mainstream for IP)

– System geometry

– System waveform

– Component measured

– Ex, Ey, Ez, Bx, By, Bz, B, dBx/dt, dBy/dt, dBz/dt & derived parameters such as Phase, Tilt (Tipper), Impedance (from E/B ratios), gradients, tensors.

• There is published experience and advice on methodology available for common targets in common backgrounds: eg volcanogenic masssivesulphides, sedimentary-exhalative lead-zinc, diamonds in kimberlites (not when I started in 1972 however)

There are still no good books or comprehensive papers that clearly, simply and correctly discuss mineral exploration using all the facets of EM. EM, being a combination of vector potential fields, waves and diffusion, is by far the most difficult discipline of classical physics. There are plenty of papers on modelling(many with dubious results) using vastly oversimplified discretization. There are a number of good case histories, but these examples cannot be generalised with confidence. These is very little published on comparing EM systems and understanding the differences between ideal and practical systems.

Magnetic fields B see currents, dB/dt fields see changing currents, E fields see mainly charge with a contribution from changing magnetic fields (mostly the primary)

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Electromagnetics of obscure targets similar to that of conventional ones

• Are you looking for one? If so:

– What are its physical properties?

– Can it be distinguished with EM? if so with which components?

– What is the target setting?

– Would this setting have a distinctive electromagnetic signature?

– Basically, what is the chance of success?

– If reasonable, try it out

• Are you interpreting data with an open mind?

PHYSICAL PROPERTIES

ATTRIBUTES OF THE GROUND

INSTRUMENTATIONAND FIELD

TECHNIQUES

GEOPHYSICAL SURVEYING AND

DATA PROCESSING

INTERPRETATION THEORY AND

ANALOG MODELLING

INTERPRETATION OF DATA

DESIRED INFORMATION

RESEARCH OPERATIONS

Palacky (1983) flowchart

with slight revision

NUMERICAL?

OTHERPOSSIBILITIES

There are many obscure targets to which EM can be applied that I have not included in previous lists. This slide is a summary of the (obvious) process to undertake for an unknown case where there are no discoverable precedents, with the twist that EM is more than the systems you are familiar with.

A tabular form of this slide is Figure 1 in Palacky, 1988, “Characteristics of Geological Targets” SEG EM volume 1, Theory

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Chrome

• Early high-carbon ferrochromium furnaces used high-grade, lumpy chromite (podiform) primarily from Rhodesia

• With increasing demand from the 1970s, production commenced from lower-grade ores (stratiform), mostly from South Africa. The alloy produced from these ores became known as charge chrome because the chromium content was lower and the carbon content, and in particular the ratio of C:Cr, was very much greater.

• This did not suit the stainless steelmakers who required as little carbon as possible entering their melts for each chromium unit and who were, therefore, having to use larger amounts of the more costly low-carbon ferrochromium to compensate.

– Podiform Chromite supply was almost essential in the 1980’s

– Zimbabwe unreliable, several significant ophiolitepodiform deposits were running out

– EM Geophysics was considered for characterisation, and I was asked to evaluate methodology for surface and boreholes (including in-mine boreholes)

Bushveld outcrop & stratiform chromite

My good picture of a 2 m high high-grade podiform chromite boulder has vanished into my filing system or been thrown out, so I invented a black blob

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Podiform Chromite• CAPSULE DESCRIPTION: Deposits of massive chromite occur as pods, lenses or layers within ophiolitic

• ultramafic rocks.

• TECTONIC SETTING: Obducted fragments of oceanic, lower crustal and upper mantle ultramafic rocks

• within accreted oceanic terranes.

• DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Formed as a primary magmatic

• differentiate during early olivine and chrome-spinel crystal fractionation of basaltic liquid at an

• oceanic spreading centre; (1) as massive to disseminated pods and lenses of chrome-spinel

• surrounded by a dunite envelope within depleted mantle harzburgite; or (2) as massive to

• disseminated cumulate layers in dunite at the base of the crustal plutonic section.

• AGE OF MINERALIZATION: Mesozoic and younger.

• HOST/ASSOCIATED ROCK TYPES: Variably serpentinized peridotite; residual mantle harzburgite;

• cumulate dunite.

• DEPOSIT FORM: Podiform, tabular lenses, irregular masses, cumulate layers. Pods and lenses typically

• occur in clusters of variable size.

• TEXTURE/STRUCTURE: Massive to disseminated, nodular (syn. leopard, grape, bean or shot ore),

• chromite net, occluded silicate, orbicular.

• ORE MINERALOGY: Chromite.

Ash, Chris (1996): Podiform Chromite, in Selected British Columbia Mineral Deposit Profiles, Volume 2 - Metallic Deposits, Lefebure, D.V. and Hõy, T,

Editors, British Columbia Ministry of Employment and Investment, Open File

1996-13, pages 109-112. Extracted from a Yukon/BC geological survey website

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Geophysics for Chromites

• Gravity common: Chromite has1.5 ton / m3 contrast with surroundings

• Fails in heavily weathered ophiolite terrain despite exhaustive terrain correction due to severe topography, variable weathering and rock component density

• In the Shetlands, only 2 of 7 drilled (from 16 target) gravity anomalies had Chromite sources

Copyright picture not in hardcopy

Parasnis, principles of applied geophysics, 1997 mentions gravity for chromites

Gravimeter prospecting for chromite in Cuba, Geophysics 10, 34 (1945); doi:10.1190/1.1437146 Hammer, Nettleton, and Hastings Gulf research & development company, Pittsburgh,

Pennsylvania

Geophysical investigation of chromite-bearing ultrabasic rocks in the Baltasound-Hagdale area, Unst, Shetland Islands. Johnson, C.E.; Smith, C.G.; Fortey, N.J.. 1980 Geophysical investigation of chromite-bearing ultrabasic rocks in the Baltasound-Hagdale area, Unst, Shetland Islands. Institute of Geological Sciences, 79pp. (WF/MR/80/035) (Unpublished)

“Economic deposits of chromite were discovered in Unst, Shetland Islands in the early part of the

nineteenth century and extraction continued intermittently until exhaustion of the known near-

surface deposits in 1945. Since it is likely that further comparable deposits exist at shallow depth,

detailed geophysical surveys employing 2 magnetic and electrical methods were carried out over

? km of the area of known mineralisation to test the feasability of detecting and delineating them.

Seven of 16 small positive gravity anomalies tested by shallow boreholes but only two were

attributed to chromite concentrations.The gravity anomalies at the other borehole sites remain

unexplained; they may be due to unidentified variations in bedrock density at depth; they may be

of weathering and the thickness of in overburden thickness.”

Case histories of the application of geophysical methods to chromite exploration in the Balkans. ALEKSANDER KOSPIRI, PETRICA KOSHO and NDOC VUKZAJ, Second Balkan Geophysical congress and exhibition. Bruga “mihal grameno” nr. 130 tirana, albania. E-mail: geofuz@ cgge.Tirana.Al

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Chromite Exploration (1980’s)

• What is conductivity of Chromite (unknown)

– Look in Keller & Frischknecht & other references….No Luck

– [ Repeated exercise with Google last month: no better luck with a lot of work ]

• Sent instructions to test with multimeter

– Answer: Lumpy chromite non-conductive

– Leopard chromite quite conductive

– (chromite in serpentinised rock)

• Eliminated B or dB/dt sensors for the massive chromite, considered E fields (Inductive Source Resistivity)

More recently, a Google search failed to produce values of conductivity for Chromite. They may exist, but not easy to find!

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Setting of ultrabasic ophiolite chromites (Lago et al)

Harzburgite is one of the peridotites

What is peridotite and dunite conductivity?

A search turned up a 1973 paper in Geophysics authored by Z. Dvorak

entitled “Electrical conductivity of several samples of olivinites, peridotites and dunites, as a function of pressure and temperature.”

At room temp:

Dunite 105 – 107 Ωm

Peridotite 106 – 108 Ωm

Cross Section from:

Lago, Rabinowicz and Nicolas, 1981, Podiform Chromite Ore Bodies, a Genetic Model. Journal of Petrology 23, 103-126

Hot conductivities from:

Dvorak, Z, 1973, Electrical conductivity of several samples of olivinites, peridotites and dunites, as a function of pressure and temperature, Geophysics 38, 14-24.

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Did survey using the Electric component of EM (ISR) since everything except weathered surface was expected to be resistive.

Electrodes

Dunite HarzburgiteChromite

In the “proof of concept”area, harzburgite was serpentenised and more conductive than Dvorak’s samples, and chromite lay at a dunite-harzburgite contact rather than within a narrow dunite

Scanned slides from a old talk on ISR where the target was not mentioned. The primary E field from a UTEM system is a square wave, and with a distant source can be considered to be uniform in the vicinity of a small target

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Vertical Hole axial (vertical) E field response. There is a response from Chromite, which appears to be the most resisitve unit detected.

Laterite (cased)

Dunite

Massive Chromite

Harzburgite

Altered Dunite

The chromite intersected in this hole was sub-horizontal, but most occurrences were actually sub-vertical in this environment

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The results of exploration following a “successful” test

• We found lots of isolated dunite pods but discovered no chromite of note

• Mine closed

• Cadillacs slowly but surely lost most of their chrome

1950’s

2000’s


1980’s

Less facetiously, the solution lay in chemical engineering and not geophysical or geological

The need for podiform chromite as flux changed radically with the advent of the argon-oxygen decarburising (AOD) and vacuum-oxygen decarburising (VOD) processes. These processes enabled the steelmakers to remove carbon from the stainless melts without excessive oxidation and losses of chromium.

So: what happened to the chrome bumpers?

OH&S issues?

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Other oxides as EM targets

• Direct Detection

– CDI Section from Manganese exploration in Australia, most manganese oxides being quite conductive. Instructive are the vertical offsets, caused not by geology but altimeter processing errors. On the greyscale, black is conductive and white resistive

• Indirect Detection

– EM37 borehole log from Renison Bell tin mine, Tasmania, where the resistive Cassiterite SnO2 is spatially associated with conductive Pyrrhotite

CDI

Altitude

Data

Axial component down-hole data

Early

Late

Off-hole anomaly

Top is Figure 29 from Macnae, EMFlow Expert User Manual (2001 edition)Example of the effect of altimeter errors

The above figure shows the effect of altimeter errors (occasional ‘drops’ of the order of 10m) on a CDI section. The historic Geotem data are smooth and the CDI discontinuities are purely a result of altimeter problems.

Bishop J.R., Lewis R.J.G., Macnae J.C. (1987) Down-hole electromagnetic surveys at Renison Bell, Tasmania. Exploration Geophysics 18, 265–277. doi:10.1071/EG987265

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A salutary example

Extracted from the work of Webb and Rowston, where the aim is to discuss errors of interpretation of obscure targets

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The misleading court case of Ernest Henry, IOCG

From Webb and Rowston, 1995

EM anomaly from a small

patch of supergene

Native Copper, not

from the IOCG

deposit itself

100 m loop

The geophysics of the Ernest Henry Cu-Au deposit (N.W.) QldM. Webb, P. Rowston

Exploration Geophysics, Vol. 26 No. 3 Pages 51 - 59, Published 1 September

1995

Book “Introduction to mineral exploration”, 1997

By Charles J. Moon, M. K. G. Whateley, Anthony M. Evans contains on p 67 a summary of the Savage-Western Mining Dispute

My involvement in the court case lay in claiming that EM was “dodgy”, and that the Sirotem anomaly was too small to be indicative of an economic resource. I was also of the opinion, based on experience, that economic sulphides were an uncommon source of EM anomalies! Usually uneconomic causes create EM responses. Other Australian geophysical experts (hired by Savage) disagreed and claimed the anomaly was certain to be sulphides, very probably economic.

WMC never revealed in court (as far as I know) that the EM anomaly source was in fact not massive sulphides, but Native Copper from supergene enrichment. This was published in the Webb and Rowston paper.

Note the location of the “LONG SECTION” is inconsistent on the maps

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Native Copper detected by EM and its location and size correctly predicted

100 m Loop

This discovery was a significant loss to WMC in sunk exploration costs, $17 million damages and legal expenses


Underlying graphics from Google Earth. I added (approx) the size of the EM target in relation to the pit. This was a classic case of misinterpretation (attributing an EM anomaly to sulphides) with a geological result way better than should have been expected from the geophysics.

Model from web:

http://www.datamine.co.uk/products/geological_products/Studio/Studio_brochure/block_modeling.htm

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Conclusions

• EM of obscure targets is fraught with danger, due to a lack of “statistics and experience”

• EM can be misinterpreted when obscure minerals are detected

• Public domain lists of measured physical properties would be jolly useful.

Been there, Done that!

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